The invention relates to a transponder for a detection system, which comprises a transmitter, for the production of a magnetic alternating field having a predetermined frequency, and a receiver, the transponder comprising a signal element made of soft magnetic material and the receiver being provided with a detector which detects the higher harmonics of the frequency of the alternating field, which harmonics are generated by placing the transponder in the magnetic alternating field, in which the signal element has a curved shape. A transponder of this type is disclosed in U.S. Pat. No. 4,074,249.
The known detection system comprises a transmitter which is composed of an alternating voltage generator and a coil unit connected thereto in order to generate a magnetic alternating field having a predetermined frequency. In addition, the detection system comprises a receiver, the receiving coil unit of which is placed opposite the transmitting coil unit of the transmitter. The receiving coil unit is connected to a detector of the receiver, which is set up to detect higher harmonics of the frequency of the alternating field. The known transponder for use in the known system comprises a carrier of insulating plastic material, on which a curved strip made of soft magnetic material is fixed.
When the transponder is located in the magnetic alternating field, it will be influenced by said field in such a way that harmonics of the frequency of the alternating field are generated, which harmonics are detected by the detector as an indication of the presence of the transponder in the magnetic alternating field.
The curved signal element used in the above known system provides a relatively good signal response over a wide range of orientation of the plane of the signal element in comparison to a straight element, but has the disadvantage that the detection is still direction-sensitive perpendicular to the abovementioned plane.
When the signal wire is positioned with its longitudinal direction perpendicular to the lines of force of the magnetic alternating field, no higher harmonics of the frequency of the alternating field are generated, or the amplitudes of said higher harmonics are too small to enable them to be detected. The detection system therefore works reliably only when the signal carrier has a specific orientation with respect to the magnetic alternating field.